Electronic arc welding stations are in widespread use and some of these stations provide for a wide range of arc welding operations, from constant-current DC arc power and low frequency pulsed arc power to high frequency pulsed or chopped arc power. It is well known in the art to provide an inductor and a flywheel diode in the circuitry of the welding station so that when the power transistor in the welding station is in the pulse-off state the inductor and the flywheel diode provide a source of current which maintains the arc alive until the power transistor is returned to the pulse-on state. For higher pulse or chopping rates the inductor provides for limiting of the arc current. However, for low frequency pulse and DC modes of operation the inductor provides little current limiting and may even saturate. The lack of current limiting causes a higher current to flow than is normally present when a high frequency pulse rate is used. Furthermore, the higher current flow can cause a significant swing in the output of the power supply which is powering the welding station. The swing in the output voltage can adversely affect the arc characteristics, the uniformity of the arc, and the quality of the weld.
To avoid this problem it is common to use a large filter capacitor at the welding station to provide additional current during the time that the power transistor is in the pulse-on state. In addition, to further reduce the voltage swing at the output of the power supply it is common to design the power transformer in the power supply to provide minimal voltage drop when the desired current is being drawn. Of course, this requires the use of larger primary windings, a larger core, and larger secondary windings so as to reduce the resistive (I.sup.2 R) losses in the transformer. However, this drives the price of the transformer upward. Therefore, a compromise is usually made in that the cost is held down by sizing (selecting the size of the wire and the number of turns) the primary and secondary conductors in the core such that the transformer will not saturate under a no-load condition and so that the swings in the output voltage cause adverse effects which are of a tolerable degree. Likewise, the size of the filter capacitor is selected as a compromise among the factors of size, weight, cost, and performance of the welding station.
Some welders, such as short arc welders, use a transformer, a rectifier, and an output impedance, such as an inductor or a resistor. These types of welders are not pulse arc welders and are complete in themselves. That is, they do not require an electronic welding station to be attached in order to perform welding operations. Examples of these types of welders are shown in the following U.S. Pat. Nos. 3,278,721; 4,117,304; and 4,251,710. In these types of welders the open circuit output voltage is selected to be high enough to provide reliable and efficient arc striking, typically 60 to 80 volts, and the transformer impedance and the output impedance are selected to cause the output voltage to drop to a practical arc sustaining voltage, such as 20 to 25 volts, once the arc has been struck.
However, pulse arc welding stations typically contain current sensing circuitry which adjusts the output pulse width in response to the output current. Also, these electronic welding stations typically give erratic or poor performance, or even cease operating, if the input voltage drops substantially. Therefore, the power supply for a pulse arc electronic welding station is typically made to be as stiff as is economically feasible.
The use of a "stiff" power supply, that is, one in which the swing in the output voltage is minimized, and the addition of economically feasible and realistically sized high-capacitance filter capacitors in the welding station improve the characteristics of the arc by stabilizing the voltage going into the welding station but also cause other problems.
In particular, it has been found that the use of such a stiff power supply and such filter capacitors causes, in the low frequency pulse mode, excessive heating of the filter capacitors, the output inductor, the power transistors, and the resistors used for current balancing when several transistors are used in parallel to achieve the desired welding current. It is believed that the cause of the excessive heating in the filter capacitors is at least partly due to the resistive losses in the capacitor which occur during the charging of the capacitor (when the power transistor is in the pulse-off state) because the stiff power supply has a low impedance and provides very little limiting of the charging current, and the discharging of the capacitor (when the power transistor is in the pulse-on state). The cause of the excessive heating in the power transistors is believed to be due to the turn-off time of the transistors. More particularly, a transistor transitions from the on-state to the off-state in a finite, non-zero amount of time. During this finite amount of time the current flow through the transistor is decreasing but the voltage across the transistor is increasing. During this time the transistor is not in either of its normal low power dissipation states: at or near saturation; or turned off. Therefore, the more stiff that the power supply is then the less the voltage will drop during the pulse-on state and therefore the larger the voltage that will be impressed across the power transistors when the power transistors are transitioning from the on-state to the off-state.